Synthetic aperture radar (SAR) systems rely on antenna designs which radiate single or dual polarized beams with well controlled radiation patterns. Such antenna designs may take the form of arrays of radiating primary elements such as patch antennae or waveguide slots. In all types of arrays, it is essential to the efficient operation of the SAR system that the radiated fields have linear frequency response, have high aperture illumination efficiency, well controlled sidelobes and good polarization purity.
In addition, for space based applications, the antenna must have low mass, compact form and high structural integrity to meet the loads imposed on the antenna during launch and subsequent operation in orbit. With all of these design requirements achieved it is possible using SAR techniques to form high resolution images of the earth's surface from orbit.
When operating the SAR at one frequency and with one polarization, imagery of useful but limited utility can be achieved. By increasing the frequency and polarization content of the SAR, it is possible to achieve a wider range of imagery, giving a significantly expanded degree of interpretation to the images.
For example, it has been found that L-band, with its longer wavelength, gives more useful imagery through foliage, while C-band gives more useful imagery of sea ice. It has also been found that horizontal polarization at both L-band and C-band is more useful for detecting the thickness of sea ice, than vertical polarization.
In U.S. Pat. No. 4,243,990, issued Jan. 6, 1981, to J. T. Nimit et al, an integrated multiband array antenna is described. The antenna illustrated in FIGS. 3 and 4 of that patent is comprised of a first array of plural parallel waveguides which contain transverse slots on one surface, so as to radiate horizontally polarized signals. Another array of coaxial waveguides is positioned parallel to the first array, between and abutting the waveguides of the first array. Longitudinal slots on the upper surface of the second array of waveguides cause radiation of signals carried by the second array, with vertical polarization. The patent states that different frequency combinations, or the same frequency, can be emitted by this structure.
The antenna described in this patent is limited to two signals, either one frequency with dual (horizontal and vertical) polarization, or two frequencies in dual polarization. However it cannot emit using two frequencies, one being in dual polarization and the other being at vertical polarization. To obtain the latter, a second antenna array on a second spacecraft is required, an expensive proposition.
In the event such an antenna array is used on one spacecraft, the resolution of images will be limited to that which can be obtained using the above described antenna array.